Problem: $\dfrac{9}{8} - \dfrac{2}{5} = {?}$
$ = {\dfrac{9 \times 5}{8 \times 5}} - {\dfrac{2 \times 8}{5 \times 8}} $ $ = {\dfrac{45}{40}} - {\dfrac{16}{40}} $ $ = \dfrac{{45} - {16}}{40} $ $ = \dfrac{29}{40}$